Polyketides are compounds synthesized from two-carbon building blocks, the .beta.-carbon of which always carries a keto group, thus the name polyketide. These compounds include many important antibiotics, immunosuppressants, cancer chemotherapeutic agents, and other compounds possessing a broad range of biological properties. The tremendous structural diversity derives from the different lengths of the polyketide chain, the different side-chains introduced (either as part of the two-carbon building blocks or after the polyketide backbone is formed), and the stereochemistry of such groups. The keto groups may also be reduced to hydroxyls, enoyls, or removed altogether. Each round of two-carbon addition is carried out by a complex of enzymes called the polyketide synthase (PKS) in a manner similar to fatty acid biosynthesis.
The biosynthetic genes for an increasing number of polyketides have been isolated and sequenced. For example, see U.S. Pat. Nos. 5,639,949, 5,693,774, and 5,716,849, all of which are incorporated herein by reference, which describe genes for the biosynthesis of soraphen. See also, Schupp et al., FEMS Microbiology Letters 159: 201-207 (1998) and WO 98/07868, which describe genes for the biosynthesis of rifamycin, and U.S. Pat. No. 5,876,991, which describes genes for the biosynthesis of tylactone, all of which are incorporated herein by reference. The encoded proteins generally fall into two types: type I and type II. Type I proteins are polyfunctional, with several catalytic domains carrying out different enzymatic steps covalently linked together (e.g. PKS for erythromycin, soraphen, rifamycin, and avermectin (MacNeil et al., in Industrial Microorganisms: Basic and Applied Molecular Genetics, (ed.: Baltz et al.), American Society for Microbiology, Washington D.C. pp. 245-256 (1993)); whereas type II proteins are monofunctional (Hutchinson et al., in Industrial Microorganisms: Basic and Applied Molecular Genetics, (ed.: Baltz et al.), American Society for Microbiology, Washington D.C. pp. 203-216 (1993)).
For the simpler polyketides such as actinorhodin (produced by Streptomyces coelicolor), the several rounds of two-carbon additions are carried out iteratively on PKS enzymes encoded by one set of PKS genes. In contrast, synthesis of the more complicated compounds such as erythromycin and soraphen involves PKS enzymes that are organized into modules, whereby each module carries out one round of two-carbon addition (for review, see Hopwood et al., in Industrial Microorganisms: Basic and Applied Molecular Genetics, (ed.: Baltz et al.), American Society for Microbiology, Washington D.C., pp. 267-275 (1993)).
Complex polyketides and secondary metabolites in general may contain substructures that are derived from amino acids instead of simple carboxylic acids. Incorporations of these building blocks are accomplished by non-ribosomal polypeptide synthetases (NRPSs). NRPSs are multienzymes that are organized in modules. Each module is responsible for the addition (and the additional processing, if required) of one amino acid building block. NRPSs activate amino acids by forming aminoacyl-adenylates, and capture the activated amino acids on thiol groups of phophopantheteinyl prosthetic groups on peptidyl carrier protein domains. Further, NRPSs modify the amino acids by epimerization, N-methylation, or cyclization if necessary, and catalyse the formation of peptide bonds between the enzyme-bound amino acids. NRPSs are responsible for the biosynthesis of peptide secondary metabolites like cyclosporin, could provide polyketide chain terminator units as in rapamycin, or form mixed systems with PKSs as in yersiniabactin biosynthesis.
Epothilones A and B are 16-membered macrocyclic polyketides with an acylcysteine-derived starter unit that are produced by the bacterium Sorangium cellulosum strain So ce90 (Gerth et al., J. Antibiotics 49: 560-563 (1996), incorporated herein by reference). The structure of epothilone A and B wherein R signifies hydrogen (epothilone A) or methyl (epothilone B) is: ##STR1##
The epothilones have a narrow antifungal spectrum and especially show a high cytotoxicity in animal cell cultures (see, Hofle et al., Patent DE 4,138,042 (1993), incorporated herein by reference). Of significant importance, epothilones mimic the biological effects of taxol, both in vivo and in cultured cells (Bollag et al., Cancer Research 55: 2325-2333 (1995), incorporated herein by reference). Taxol and taxotere, which stabilize cellular microtubules, are cancer chemotherapeutic agents with significant activity against various human solid tumors (Rowinsky et al., J. Nat. Cancer Inst. 83: 1778-1781 (1991)). Competition studies have revealed that epothilones act as competitive inhibitors of taxol binding to microtubules, consistent with the interpretation that they share the same microtubule-binding site and possess a similar microtubule affinity as taxol. However, epothilones enjoy a significant advantage over taxol in that epothilones exhibit a much lower drop in potency compared to taxol against a multiple drug-resistant cell line (Bollag et al. (1995)). Furthermore, epothilones are considerably less efficiently exported from the cells by P-glycoprotein than is taxol (Gerth et al. (1996)). In addition, several epothilone analogs have been synthesized that have a superior cytotoxic activity as compared to epothilone A or epothilone B as demonstrated by their enhanced ability to induce the polymerization and stabilization of microtubules (WO 98/25929, incorporated herein by reference).
Despite the promise shown by the epothilones as anticancer agents, problems pertaining to the production of these compounds presently limit their commercial potential. The compounds are too complex for industrial-scale chemical synthesis and so must be produced by fermentation. Techniques for the genetic manipulation of myxobacteria such as Sorangium cellulosum are described in U.S. Pat. No. 5,686,295, incorporated herein by reference. However, Sorangium cellulosum is notoriously difficult to ferment and production levels of epothilones are therefore low. Recombinant production of epothilones in heterologous hosts that are more amenable to fermentation could solve current production problems. However, the genes that encode the polypeptides responsible for epothilone biosynthesis have heretofore not been isolated. Furthermore, the strain that produces epothilones, i.e. So ce90, also produces at least one additional polyketide, spirangien, which would be expected to greatly complicate the isolation of the genes particularly responsible for epothilone biosynthesis.
Therefore, in view of the foregoing, one object of the present invention is to isolate the genes that are involved in the synthesis of epothilones, particularly the genes that are involved in the synthesis of epothilones A and B in myxobacteria of the Sorangium/Polyangium group, i.e., Sorangium cellulosum strain So ce90. A further object of the invention is to provide a method for the recombinant production of epothilones for application in anticancer formulations.